Abstract： In this paper, it is discussed that when monochromatic light incident normally on an arbitrarly making directional biaxial crystal. The formulas can be deduced for propagating directions of polarized light and the transmittance—reflectances of incident components. An important conclusion isobtained that directions of polarization are always on the refractive planes and the two refractive planes are perpendicular with ecch other. And finally, the numerical results are plotted in curvilineal diagrams for NaNO2, it is seen that the changes of the emergent points of polarized light and that of the reflectances are obvious as the crystal making directions change. These results offer a new possibility for determination of crystal optical constants.